IB Physics SL E.1.6 Spectra & composition

Practise explaining how astronomers use stellar spectra, especially line positions, to infer which elements are present in stars.

Syllabus
First assessment 2025
Objective
—
Level
SL

Exam points

  • explain that gases in a star produce absorption lines, then compare observed wavelengths with laboratory spectra
  • use matching spectral lines to justify the presence of a named element such as helium in the Sun

E.1.6—Spectra and composition question 1

[Maximum number: 2]

The diagram shows a simplified energy-balance model for the Earth surface–atmosphere system.

Figure for Question E.1.6—Spectra and composition question 1 — IB Physics SL

The following data are given:

 Average albedo of Earth =0.30 Average global temperature of the surface =288 K Average Earth-Sun distance =1.5×1011 m\begin{aligned} \text { Average albedo of Earth } & =0.30 \\ \text { Average global temperature of the surface } & =288 \mathrm{~K} \\ \text { Average Earth-Sun distance } & =1.5 \times 10^{11} \mathrm{~m} \end{aligned}

The primary energy source of the Sun is the proton-proton (p-p) chain of fusion reactions. Four protons and two electrons produce a helium nucleus together with neutrinos and gamma photons. The overall reaction is:

411p+2−10e→24He+200ve+4γ4{ }_{1}^{1} \mathrm{p}+2{ }_{-1}^{0} \mathrm{e} \rightarrow{ }_{2}^{4} \mathrm{He}+2{ }_{0}^{0} v_{e}+4 \gamma

Outline how the presence of helium in the Sun can be confirmed empirically.

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